Screenwriter has recently started inserting about 10 blank spaces after every period while writing in ACTION. I've checked everything I can think of in formatting & preferences, but no fix. Anybody have a clue?

Did you check whether the 'Auto-Space sentences' option is set correctly? This is the only reason I can think of that screenwriter behaves this way. To check it, go to Tools->Options->Spelling (Tab) in Screenwriter 6 or File->Program Options->Spelling (Category) in MMS4. There is an option called 'Auto-Space sentences' that tells the program to insert a variable number of spaces after each sentence. Try removing the check mark or changing the value to 1 (or 2, however you like it).